Thanks for the link. I have now added text/xml and application/xml mime types to the mimeTypes.rdf file in my profile. The xml type now shows up in the Tools\Options\Applications and I've set the handler to Notepad++ but when I click on an XML link it still opens in the default xml viewer. Is the default viewer overriding this setting or am I missing something? Thanks
–
ChrisBMay 11 '11 at 14:24

Holy cow! This issue drove me CRAZY all evening. I finally figured it out. I realize the original question was long ago, but I want this answer on the internet, so here goes:

Instead of adding an entry for XML, I had to update the extensions list for text/plain. Here is what the text/plain entry now looks like in my mimeTypes.rdf file. The file is located in the firefox profile folder, see firefox troubleshooting page (type about:support into the navigation bar) and under "Profile Folder" click on "Show Folder" then edit the mimeTypes.rdf file with notepad++.